bumpluk solution
Suspect faulty (Murphied) IRS triplex reconnection/re-racking by grndcrew following attempts to isolate a faulty centre IRS for MEL. #One hooked incorrectly to centre (dead), #two hooked up to #one etc - and circumstance enabled by one IRS being MEL'd out of the picture. (but the one thus disabled by the MEL attempt was probably a functioning one). The perils of inadvertent interchange of units or their connections....
Fault probably set off by TOGA press (for IRS reset) on runway line-up....and real conflict started once aircraft turned for SID ( IGRIS 1A departure) and corrective voltages (to precess gyros etc) were being applied (but all of these now being to the wrong place, i.e. wrong corrector coils). Systems failures followed because all the required input data-feeds were either not forthcoming or were being actively rejected, not crc-checking or wrong polarity.
If it is possible to murphy an installation, sooner or later someone will do it. Having all the leads off and being in a rush to replace two and MEL the third sounds like a classic "departure rush" setup.